1. Field of the Invention
The present invention relates to a system for remotely locating pairs of intermediate amplifying circuits which are included between a monitoring equipment and a remote equipment of a bidirectional PCM transmission medium.
More particularly, the invention concerns a remote locating system in which the remote location of intermediate amplifying circuit pairs is performed step-by-step, namely with no special addressing signal peculiar to each pair. No auxiliary means for transmitting the remote locate signals are required other than the communication medium normally carrying the PCM data signals and mixers for superimposing frequencies. Such a system does not have to employ intermediate amplifying circuit means, such as repeater input and output power-separation filters, frequency filters and demodulators or a circuit for detecting an address peculiar to the particular pair. The result is simplicity of the remote locate circuit which is associated with each pair of intermediate amplifying circuits, together with relatively low manufacturing costs, reduced current consumption and excellent reliability.
Remote location as employed in the invention is based on the detection of interruptions in the PCM signal, which interruptions do not normally occur in the digital signal carried by the forward channel of the link from the monitoring equipment to the remote equipment. These interruptions have characteristics that adhere to the transparency rules set forth in the corresponding notes and recommendations distributed by international organizations in the telephone transmission field.
2. Description of the Prior Art
A known remote location procedure with no addressing is described in the article by Reginhard Pospischil entitled "Digital System DS 34CX for Transmitting 34 M bit/s Signals on Coaxial Pairs", published in Telcom report 2 (1979) Special Issue "Digital Transmission", pages 100 to 104, in particular under paragraph "Monitoring and fault Locating" or in U.K. Pat. No. 1,551,172, published Aug. 22, 1979.
The remote location procedure described in these two documents makes use of first and second interruptions of predetermined lengths as both a remote locate sequence, also termed "loop closure signal" of a pair intermediate circuits and a locate end interruption.
The remote locate sequence is made up of a single interruption, i.e. a series of several consecutive digits in the zero state. The predetermined number of these zero digits is equal to an integral multiple of the greatest number of zero digits likely to be contained in the PCM data signal normally conveyed in the link. For example, when the code used in the link is the HDBn bipolar code, the remote locate sequence is composed of an integral number of n+1 digits in the zero state. All the remote locate sequences are identical.
In order to loop the two intermediate amplifying circuits of a pair to the monitoring equipment, an interruption, referred to as a "preparation signal" is emitted by the monitoring equipment so as to energize the remote locate circuits of all the pairs. A number of identical remote locate sequences equal to the selected pair rank in the link is then emitted on the forward channel from the monitoring equipment to achieve a connection between two amplifying circuits of said pair through the respective loop-back path. The time interval between two remote locate sequences or two first interruptions lasts a predetermined length of time, approximately 100 .mu.s, and is filled with the digits of a conveyed digital signal such as a test signal.
To break the loop, a second interruption, or locate end interruption, is transmitted on a forward line channel by the monitoring equipment. The remote locate circuit associated with the previously looped pair detects this second interruption and controls switchable looping and unlooping means so as to open the loop-back path. The second interruption is also composed of a series of consecutive zero-state digits numbering more than those contained in a remote locate sequence.
During the routing of the PCM signal, it is known that interference interruptions may be present due, for example, to factors outside the link such as lightning or factors within the link, such as repeaters functioning abnormally for a short period of time. These interference interruptions can be roughly as wide as the remote locate sequences in the previously known procedure. In this case, the interference interruptions can cause untimely looping which upsets not only the PCM link transmission in normal operation, but also the various remote locations during a remote locate and test procedure.